The present invention relates to a purification method of a closed water area in a bay, a lake, a marsh, a dam lake, etc., in which water is dead and less movable, and to an apparatus for use in the purification.
In a closed water area in a bay, a sea water culture has been carried out for a long time. Since the water in a bay is less exchangeable with a water of open sea, the water in a bay is polluted due to self-contamination. Also, in a lake, a marsh and a dam lake, the water is polluted due to inflow and accumulation of domestic waste water. Since the water in a closed water area is less exchangeable as mentioned above, the water is thermally stratified most of a year as shown in FIG. 4. The stratification of the water in a closed water area occurs due to a density gradient between a surface layer water, a middle layer water and a bottom layer water. The density gradient is caused by a decrease in the density of the surface layer water due to a heat of solar radiation and a temperature difference between the water and air contacting the water surface. The thermal stratification of FIG. 4 consists of, from the bottom, a bottom layer 52 having a low temperature and a high density, a thermocline layer 53 thereon in which the temperature abruptly changes, and a surface layer 54 having a high temperature and a low density. In addition to the stratification due to the temperature difference, a stratification due to a difference in dissolved oxygen (DO) contents may occur.
The atmospheric temperature and the sunshine intensity changes daily and annually, and the water is stratified depending on the daily change and annual change. In FIG. 4, a dashed curve 41 shows a temperature distribution of a surface layer water in the daytime, and a curve 42 shows a temperature distribution in the night. The temperature distribution in the night is nearly the same as a temperature distribution caused by the annual differences in sunshine intensity and the atmospheric temperature. In the temperature distribution curve 42, the portion 53 in which the temperature changes abruptly is referred to as a primary thermocline layer. A surface layer 54 referred to as a secondary thermocline layer has a higher temperature which changes abruptly, and occurs due to the daily differences in sunshine intensity and the atmospheric temperature.
Since the bottom layer water has a low temperature and the sunlight does not reach the bottom layer, the photosynthesis by phytoplanktons does not take place there and oxygen is not released to the bottom layer. Also, since the stratification prevents the convection of the water, the dissolved oxygen (DO) in the surface layer is not transferred to the bottom layer. Therefore, the bottom layer water forms an oxygen-deficient water mass having a DO content of nearly zero.
Contrary to the bottom layer, since the surface layer has a higher temperature and receives a sufficient amount of sunshine, the photosynthesis by phytoplanktons vigorously occurs therein and oxygen in the air contacting the water surface is also dissolved into the surface layer water, the DO content of the surface layer water reaches 8-10 ppm, in some cases, reaches a supersaturated amount of about 30 ppm.
In the stratifying phase (summer season in which a large temperature difference is created between the surface layer, the middle layer and the bottom layer), the oxygen-deficient water mass on the mud of the sea-bottom or lake-bottom becomes a density current (thin laminar flow) and flows into a recessed or depressed portion of the sea-bottom or lake-bottom and accumulated there. In the recessed portion, organic substances such as dead bodies of phytoplanktons are sedimented and accumulated to form the mud. In the oxygen-deficient water mass having a low DO concentration, anaerobic bacteria actively produce from the mud nutritive salts of organic acids, ammonia, phosphoric acid, etc. and poisonous gases such as hydrogen sulfide to make the oxygen-deficient water mass more eutrophic.
In late summer or autumn (from September to October in Japan) in which the atmospheric temperature sometimes becomes lower than the water temperature, the thermal stratification of the closed water area is gradually disappeared to enter upon a circulating phase. At be beginning of the circulating phase, the oxygen-deficient water mass in the bottom layer occasionally rises near the surface layer by some external causes such as a transitory wind to kill the cultured fishes, etc. in a short time. Also, since the oxygen-deficient water mass is rich in the nutritive salts, the oxygen-deficient water mass rising into the surface layer receiving a strong sunlight causes an explosive growth of phytoplanktons such as water bloom, thereby preventing the good use of the water.
The temperature distributions in the depth direction of the same closed water area (Ohfunato Bay in Iwate-ken, Japan) are shown in FIG. 5A for the stratifying phase in summer (August) and in FIG. 5B for the circulating phase in late autumn (November). FIGS. 6A and 6B schematically show the distribution of the dissolved oxygen concentration in the water depth direction of the same closed water area in the same season as in FIGS. 5A and 5B. In summer, both the water temperature and the dissolved oxygen concentration change consecutively from the surface layer to the deep layer to form a stable stratification. However, in late autumn, the stratification is disappeared and the distributions of the water temperature and the dissolved oxygen concentration become unclear. In summer in which the closed water is thermally stratified, the oxygen-deficient water mass having a DO concentration of 4 ppm (mg/liter) is likely to be formed particularly in a recessed portion of the sea-bottom and lake-bottom.
To avoid the damage due to the rise of the oxygen-deficient water mass into the surface layer, proposed is a method for disappearing the oxygen-deficient water mass in the bottom layer, in which the bottom layer water containing the oxygen-deficient water mass is pumped up and discharged into the surface layer to mix the oxygen-deficient water mass with the surface layer water to diffuse the oxygen-deficient water into the oxygen-rich water of the surface layer of the closed water area. In another method, the oxygen-deficient water mass is disappeared by discharging a sucked surface layer water into the bottom layer to mix the surface layer water and the bottom layer water.
As a means for practicing the above method, Japanese Patent Laid-Open No. 5-309395 discloses agitating aeration apparatuses shown in FIGS. 7 and 8.
The agitating aeration apparatus of FIG. 7 comprises a float 101 and a pump 102 vertically suspended from the float 101. The pump 102 comprises a discharge casing 103, an intake casing 104, an electric motor 105, and an impeller 106. An opening 103a of the discharge casing 103 is positioned in the surface layer water, and an opening 104a of the intake casing 104 is positioned in the bottom layer water. Upon rotating the impeller 106, the bottom layer water is sucked through the opening 104a of the intake casing 104 as shown by arrows A, A, and discharged horizontally from the opening 103a of the discharge casing 103 in the direction shown by arrows B, B.
The agitating aeration apparatus of FIG. 8 comprises a float 111 and a pump 112 vertically suspended from the float 111. The pump 112 comprises an intake casing 113, a discharge casing 114, an electric motor 115, and an impeller 116. An opening 113a of the intake casing 113 is positioned in the surface layer water, and an opening 114a of the discharge casing 114 is positioned in the bottom layer water. Upon rotating the impeller 116, the surface layer water is sucked through the opening 113a of the intake casing 113 as shown by arrows C, C, and discharged horizontally from the opening 114a of the discharge casing 114 in the direction shown by arrows D, D.
However, the known apparatus has the following disadvantages.
In the agitating aeration apparatus of FIG. 7, since the bottom layer water of low temperature is discharged into the surface layer water of high temperature, the discharged bottom layer water flows downward to cause convection currents in the vertical direction, thereby failing to be sufficiently mixed with the surface layer water. In the agitating apparatus of FIG. 8, the convection currents in the opposite direction to the above are caused. When the convection currents reach a steady state, a stable flow pattern of convection currents and a stable DO diffusion flume are formed around the agitating aeration apparatus thereby creating a DO concentration distribution in which DO is saturated in the central portion of the agitating aeration apparatus and the DO concentration decreases with increasing distance from the apparatus. Namely, the surface layer water and the bottom layer water are mixed and diffused only in a narrow portion around the agitating aeration apparatus, and as a result thereof, DO is transferred only to a limited area.
Japanese Patent Laid-Open No. 7-136637 discloses an agitating apparatus 121 as shown in FIG. 9, which comprises an upper agitator of top-sucking and side-discharging type and a lower agitator of bottom-sucking and side-discharging type. The upper agitator has a vertical cylinder 122, at the center of which a submersible electric mixer 123 equipped with a rotating impeller is disposed, and a discharging means 127 disposed below the cylinder 122 and comprising two horizontal plates 124, 125 and a projection 126 for regulating the water flow. The lower agitator has a similar construction to that of the upper agitator and is attached to the lower surface of the upper agitator with upside down. The surface layer water sucked from a top opening of the cylinder 122 is accelerated by the mixer 123 to be discharged from the discharging means 127 in the horizontal directions. Likewise, the bottom layer water sucked from a bottom opening of the cylinder 122xe2x80x2 is accelerated by the mixer 123xe2x80x2 to be discharged from the discharging means 127xe2x80x2 in the horizontal directions. Thus, in the agitating apparatus of Japanese Patent Laid-Open No. 7-136637, the surface layer water of low density and high temperature is discharged from the upper discharging means 127, and the bottom layer water of high density and low temperature is discharged from the lower discharging means 127xe2x80x2, respectively in the horizontal directions. Since the discharged surface layer water moves upward and the discharged bottom layer water moves downward due to the density difference between them to independently forms respective convection currents, the surface layer water and the bottom layer water are not sufficiently mixed and a broad horizontal diffusion of the discharged water does not occur.
Accordingly, an object of the present invention is to solve the disadvantages in the known agitating apparatus, and provide a method in which the surface layer water and the bottom layer water are efficiently mixed to each other and the mixed water is rapidly diffused over a wide range of the closed water area, and an apparatus practicing the method.
As described above, in the closed water area in lake, marsh, dam lake, bay, etc., the surface layer water is warmed by the heat of sunshine and the air contacting the water surface to cause the temperature gradient in the water of closed area. Therefore, the water of closed area is thermally stratified most of the year. As shown in FIG. 4, the thermal stratification comprises, from the bottom, a deep layer (bottom layer) 52 of low temperature and high density, a thermocline layer 53 thereon in which the temperature changes abruptly with respect to the depth, and a surface layer 51 of high temperature and low density. In the daytime, a secondary thermocline appears due to the difference in the sunshine intensity and the atmospheric temperature between daytime and night. Although depending on the water depth, the shape of the bottom, the atmospheric temperature, the sunshine intensity, etc., the surface layer 51 has a temperature of 18-23xc2x0 C. and is formed at surface to a depth of about 4 m, the thermocline layer 53 has a temperature of 16-19xc2x0 C. and is formed at a depth of about 4-6 m. The deep layer 52 is a water layer with a lower temperature formed under the thermocline layer. The secondary thermocline is formed at a depth of about 1.5-2 m and is rich in phytoplanktons and DO.
Water flowing into the closed water area moves along the bottom thereof, and when reaches a water layer with the same density, the water diffuses into the water layer in the form of a laminar current over a wide range of the closed water area. Such a diffusion is called as a density current diffusion.
The inventors have found that when a mixture of the bottom layer water forming an oxygen-deficient water mass and the surface layer water forming an oxygen-rich water mass prepared in a mixing/diffusion apparatus is discharged nearly horizontally into a specific water layer in the thermal stratification, the discharged mixed water forms a density current to diffuse rapidly over a wide range of the closed water area. The present invention has accomplished by this finding.
Thus, in a fist aspect of the present invention, there is provide a method for purifying a closed water area by mixing and diffusing, which comprises the steps of (1) sucking a surface layer water and a bottom layer water of the closed water area separately; (2) mixing the surface layer water and the bottom layer water to prepare a mixed water; and (3) discharging the mixed water into a water layer having a temperature nearly the same as that of the mixed water to form a density current of the mixed water, thereby diffusing the mixed water radially in nearly the horizontal direction through the water layer, the water layer constituting one of stratified layers of the closed water area comprising a plurality of water layers having different temperatures and different densities.
In a second aspect of the present invention, there is provided a mixing/diffusing apparatus comprising (1) an upper intake for sucking a surface layer water of a closed water area disposed in a vertically upper portion of the apparatus, (2) a lower intake for sucking a bottom layer water of the closed water area disposed in a vertically lower portion of the apparatus, (3) a sucking means for sucking at least one of the surface layer water and the bottom layer water, and (4) a mixing/discharging means for mixing and discharging the sucked surface layer water and bottom layer water disposed at a vertically intermediate portion between the upper intake and the lower intake.